The US moved to assert its leadership and save the UN climate talks in Copenhagen from collapse today, saying it was ready to support a $100bn (£62bn) fund to shield poor countries from the ravages of climate change.

But speaking at the conference, Hillary Clinton, the US secretary of state, warned developing countries that the finances would only flow if their leaders signed up to a strong global warming deal at the summit.

Zenawi acknowledged that his proposal would disappoint some in Africa. But he said: "My proposal dramatically scales back our expectation of the level of funding in return for more reliable funding and a seat at the table in the management of such fund."

Standing with reporters when the news broke, the UK prime minister's official spokesman was surprised by the timing of Clinton's announcement, despite the fact that one of Gordon Brown's chief negotiators, Jon Cunliffe, had been on the phone with his American counterpart overnight.

"Obama said he wanted to be as helpful as he could but was concerned about public opinion at home," said one official. Another added: "This is a very serious move by the Americans. We were waiting for it".

Clinton also made it clear that America would not budge on its demand for greater accountability from rapidly emerging economies like China and Brazil that they are living up to whatever pledges they make to cut emissions.

Without such transparency, she said, there would be no deal. And without a deal, there would be no money for African and low-lying countries that have the most to lose from rising sea-levels brought by climate change.

Even as 115 world leaders began arriving to put their personal imprint on a deal, the summit hosts were admitting they had failed to broker an agreement. Informal talks on finance and the overall format of the deal were continuing yesterday, but the spokesman for the bloc of African countries warned about the perils of pushing poor countries to a cosmetic deal at any cost.

"Any bad solution for the developing countries is worse than no deal at all in particular for Africa and for the developing countries," said Kamel Djemouai, spokesman for the African group. "Those who are forcing the process who are trying to jeopardise what we are doing I am not sure humanity will forgive them at least for the next 50 years."

The chaotic end game to the negotiations could mean that world leaders only have time to hastily paper over a face-saving agreement.

In a story headlined Denmark gives up, the influential Berlingske newspaper quoted a senior source in the host delegation, saying the failure was a monumental disappointment to the Danes.

"During the whole process, the problem is that this is a huge puzzle where all the pieces had to fall in place at the same time. But to do that, the countries had to make a serious effort and they have been unwilling to do so," the source was quoted as saying.

However, Denmark could try to revive the process by formally introducing a version of a negotiating draft from last week and imposing it on the summit. However, the draft – the Danish text leaked to the Guardian last week – has infuriated developing countries, and its re-entry could trigger chaos.